Neville Longbottom: "This is the only way in or out now. The grounds are crawling with Death Eaters and Dementors."

— Harry Potter is surprised that the passage leads to the Room of Requirement, and Neville Longbottom explains that[src]

Hogwarts Castle contains several secret passageways. At least one of these was also discovered by Fred and George Weasley. Several passages are known by the majority of the students. Many, however, are only known by a few students. It is often said that Argus Filch, a caretaker at Hogwarts, knew the location of every passage in the school. The Marauders also knew at least most of passages, and included seven of them on the map they created of the school.

A passage behind a mirror on the fourth floor, which was caved in as of 1993. It leads to Hogsmeade, but where in Hogsmeade it leads to is unknown. Sirius mentioned that it was large enough for an organisation to hold a meeting inside.

A passage beneath a one-eyed witch statue by the stairs to the Defence Against the Dark Arts classroom, leading to the cellar of Honeydukes. Tapping the hump with a wand and speaking aloud the word Dissendium to the witch accesses this passage, the hump on the statue then opens and reveals the hidden passageway.

Peeves is the one who smashed one of the two vanishing cabinets, after being convinced to do so by Nearly Headless Nick